Carbon nanotube supported platinum-palladium nanoparticles for formic acid oxidation

Pt, Pd and Pt_xPd_y alloy nanoparticles (Pt₁Pd₁, Pt₁Pd₃, atomic ratio of Pt to Pd is 1:1, 1:3, respectively) supported on carbon nanotube (CNT) with high and uniform dispersion were prepared by a modified ethylene glycol method. Transmission electron microscopy images show that small Pt and Pt_xPd_y nanoparticles are homogeneously dispersed on the outer walls of CNT, while Pd nanoparticles have some aggregations and comparatively larger particle size. The average particle sizes of Pt/CNT, Pt₁Pd₁/CNT, Pt₁Pd₃/CNT and Pd/CNT obtained from the Pt/Pd (2 2 0) diffraction peaks in the X-ray diffraction patterns are 2.0, 2.4, 3.1 and 5.4 nm, respectively. With increasing Pd amount of the catalysts, the mass activity of formic acid oxidation reaction (FAOR) on the CNT supported catalysts increases in both cyclic voltammetry (CV) and chronoamperometry (CA) tests, although the particle size gets larger (thus, the relative surface area gets smaller). The CV study indicates a ‘direct oxidation pathway’ of FAOR occurred on the Pd surface, while on the Pt surface, the FAOR goes through ‘CO_ads intermediate pathway’. Pd/CNT demonstrates 7 times better FAOR mass activity than Pt/CNT (2.3 mA/mgPd vs. 0.33 mA/mgPt) at an applied potential of 0.27 V (vs. RHE) in the CA test.     

Size-dependent antimicrobial properties of sugar-encapsulated gold nanoparticles synthesized by a green method

The antimicrobial properties of dextrose-encapsulated gold nanoparticles (dGNPs) with average diameters of 25, 60, and 120 nm (± 5) and synthesized by green chemistry principles were investigated against both Gram-negative and Gram-positive bacteria. Studies were performed involving the effect of dGNPs on the growth, morphology, and ultrastructural properties of bacteria. dGNPs were found to have significant dose-dependent antibacterial activity which was also proportional to their size. Experiments revealed the dGNPs to be bacteriostatic as well as bactericidal. The dGNPs exhibited their bactericidal action by disrupting the bacterial cell membrane which leads to the leakage of cytoplasmic content. The overall outcome of this study suggests that green-synthesized dGNPs hold promise as a potent antibacterial agent against a wide range of disease-causing bacteria by preventing and controlling possible infections or diseases.     

Size-dependent melting of polycyclic aromatic hydrocarbon nano-clusters: A molecular dynamics study

The size-dependent melting behaviour of clusters of polycyclic aromatic hydrocarbon (PAH) molecules is studied computationally using the isotropic PAHAP potential (Totton et al., 2012, Phys Chem Chem Phys, 14, 4081–4094). The investigation aims to shed light on the understanding of the liquid-like behaviour of PAH clusters. Detailed molecular dynamic (MD) simulations are performed to investigate the size-dependent melting of two representative homogeneous PAH clusters composed of either pyrene (C₁₆H₁₀) or coronene (C₂₄H₁₂) molecules. The evolution of the intermolecular energy and the Lindemann index are used to estimate the melting points of individual nano-clusters. The results from the MD simulations show that individual PAH molecules within nano-clusters are highly mobile below typical flame temperatures. A detailed morphological investigation of coronene₅₀₀ further reveals that the coronene clusters evolve from a columnar particle in the solid phase to an irregular spherical particle in the liquid phase. In contrast, no such evolution is observed for pyrene₃₀₀ which remains in a spherical configuration. The nano-cluster reduced melting temperature decreases with decreasing particle size following a linear relation with reciprocal size. The melting process of these clusters starts from the surface and the liquid layer grows inwards with increasing temperature.     

Size-dependent properties of magnetic iron oxide nanocrystals

The fine control of iron oxide nanocrystal sizes within the nanometre scale (diameters range from 2.5 to 14 nm) allows us to investigate accurately the size-dependence of their structural and magnetic properties. A study of the growth conditions of these nanocrystals obtained by thermal decomposition of an iron oleate precursor in high-boiling point solvents has been carried out. Both the type of solvent used and the ligand/precursor ratio have been systematically varied, and were found to be the key parameters to control the growth process. The lattice parameters of all the nanocrystals deduced from X-ray diffraction measurements are consistent with a structure of the type Fe3-xO4, i.e. intermediate between magnetite and maghemite, which evolves toward the maghemite structure for the smallest sizes (x=1/3). The evolution of the magnetic behavior with nanoparticle sizes emphasizes clearly the influence of the surface, especially on the saturation magnetization Ms and the magneto-crystalline anisotropy K. Dipolar interactions and thermal dependence have been also taken into account in the study on the nanoscale size-effect of magnetic properties.     

Description of melting of aluminum nanoparticles

A new physicomathematical model is proposed for describing the process of melting of aluminum nanoparticles, which takes into account the dependences of thermophysical variables on the temperature and particle size obtained by the molecular dynamics method. The study is performed for samples with spherical, cylindrical, and plane symmetry. The times of melting of aluminum nanoparticles are found as functions of the nanoparticle radius and ambient temperature. Two-front melting modes are observed for the first time; these modes are the result of the scale factor in the dependence of the melting temperature on the particle size.     

On the melting temperature of nanoparticles

A theoretical explanation is offered for the possibility of melting a nanoparticle at a higher temperature as compared to the melting temperature of a macroscopic sample of the same chemical composition. The special attention is focused on the analogy between the temperature range ΔT _m of melting of nanoparticles and the softening range for glasses.     

Electrochemical preparation of iron cuboid nanoparticles and their catalytic properties for nitrite reduction

Iron cuboid nanoparticles supported on glassy carbon (denoted nm-Fe/GC) were prepared by electrochemical deposition under cyclic voltammetric (CV) conditions. The structure and composition of the Fe nanomaterials were characterized by scanning electron microscopy (SEM), selected area electron diffraction (SAED), X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX). The results demonstrated that the Fe cuboid nanoparticles are dispersed discretely on GC substrate with an average size ca. 171 nm, and confirmed that the electrochemical synthesized nanocubes are single crystals of pure Fe. The catalytic properties of the Fe cuboid nanoparticles towards nitrite electroreduction were investigated, and enhanced electrocatalytic activity of the Fe nanocubes has been determined. In comparison with the data obtained on a bulk-Fe electrode, the onset potential of nitrite reduction on nm-Fe/GC is positively sifted by 100 mV, and the steady reduction current density is enhanced about 2.4-3.2 times.     

Polyaniline stabilized highly dispersed Pt nanoparticles: Preparation,characterization and catalytic properties

Platinum–polyaniline (2%Pt/PANI) composites are prepared using three precursor solutions of H₂PtCl₆ differing in the pH and thus in the type of dominating chloro–aqua or chloro–hydroxo complexes of Pt(IV). Various characterization techniques (X-ray diffraction, FTIR and X-ray photoelectron spectroscopy, and electron microscopy; TEM, HRTEM) were employed in order to elucidate the role of platinum precursor composition in the state of Pt-species formed in the Pt/PANI composites. In all three as-received Pt/PANI composites, platinum in two valence states Pt(IV) and Pt(II) was observed thus showing redox reaction under preparation of composites. The reduction of Pt(IV) is much easier in the precursor solution of alkaline pH, i.e. when the Pt(IV) complexes comprising aqua and/or hydroxo ligands are the dominating ones. Under these conditions the content of Pt(II)-species is ca. 7× higher than that of Pt(IV) ones. Reduction of as-prepared composites by NaBH₄ gives 2%Pt/PANI catalysts of well-dispersed Pt-metal particles of size ranging from 1 nm up to 4 nm. The average diameters (2.24 nm) of Pt-particles was slightly smaller in 2%Pt/PANI catalyst prepared using H₂PtCl₆ solution of acidic pH. Catalytic properties of Pt/PANI composites are tested in liquid phase hydrogenation of alkyne-type reactant, 2-butyne-1,4-diol (B3-D) and in gas-phase hydrodehalogenation of CCl₄. In the former, a classical example of alkyne to alkene and finally alkane hydrogenation, a catalytic behavior of Pt/PANI composites in terms of activity and selectivity was found to be similar to that observed for typical inorganic carrier supported Pt-catalysts. CCl₄ hydrodehalogenation on Pt/PANI leads initially to the formation of chloroform, which is the desired reaction product. However, in the course of reaction liberation of HCl brings about large changes, both in overall activity, as well as the selectivity, giving large amounts of dimeric C₂ products, especially C₂Cl₆. It is speculated that these changes follow from reprotonation of PANI which acquires Brönsted acidity used in dimerization of CCl₃ radicals.     

Size-dependent direct electrochemical detection of gold nanoparticles: application in magnetoimmunoassays

The effect of the AuNPs size, ranging from 5 nm to 80 nm, on the electrochemical response of screen-printed carbon electrodes (SPCEs) used as electrochemical transducers is investigated for the first time. A simple hydrodynamic modelling and calculation at the nanoscale level is applied so as to find the effect of the size of AuNP upon the electrochemical response. The results show that the best electrochemical response for AuNP suspension for the same concentration of total gold is obtained for the 20 nm sized nanoparticles. It is concluded that the Brownian motions avoid a better response for smaller AuNPs that should in fact be related with the best electrochemical signal due to their higher surface area. Finally, the size effect is studied for AuNPs acting as electroactive labels in an immunosensor that employs magnetic beads as platforms of the bioreactions. The best response for the 5 nm AuNPs in this case is due to the fact that in the immunosensing conditions the Brownian motions are minimized because the AuNPs contact with the electrotransducer surface is induced by the immunoreaction and the fast magnetic collection of the nanoparticles used as antibody labels upon application of a magnetic field.     

Microfluidic synthesis of ultrasmall Co nanoparticles over reduced graphene oxide and their catalytic properties

We prepared a one-stage microfluidic-based method for continuous synthesis of cobalt (Co) nanoparticles over reduced graphene oxide (rGO) to produce Co/rGO composites. These were generated by the coreduction of Co²⁺ ions and GO with NaBH₄ which was confined within discrete aqueous plugs segmented by octane as continuous phase. Owing to the excellent transfer properties from recirculation in these plugs, ultrasmall Co nanoparticles were distributed homogeneously on the GO sheets without using any surfactants. As compared to batch methods, the average size of Co nanoparticles and the relative standard deviation decreased from 4.0 ± 1.42 nm and 35.9% to 2.0 ± 0.45 nm and 22.6%, respectively. The as-prepared Co/rGO composites exhibited superior activity towards the catalytic reduction of p-nitrophenol to p-aminophenol with NaBH₄ compared with Co nanoparticles and rGO; this enhanced activity could be attributed to the synergistic effect between Co nanoparticles and rGO.     

Size-dependent catalytic activity of copper nanoclusters for oxygen electroreduction in alkaline solution

By using 2-mercapto-5-n-propylpyrimidine (MPP) as capping ligands, copper nanoclusters with different core sizes were prepared using a chemical reduction method. The as-prepared copper nanoclusters were loaded onto a glassy carbon electrode and their size effect on the electrocatalytic activity towards the oxygen reduction reaction (ORR) was investigated with electrochemical techniques in alkaline electrolyte. Cyclic voltammetric (CV) studies showed that the onset potential of ORR on smaller copper nanoclusters is more positive than that on larger copper nanoclusters. Compared to the larger clusters, higher current density of ORR was obtained using the smaller copper nanoclusters. These CV results indicate that the smaller Cu nanoclusters exhibit higher catalytic performance for ORR. In rotating-disk voltammetric studies, ORR on the synthesized MPP monolayer-protected copper nanoclusters is mainly dominated by a two-electron transfer pathway to produce H₂O₂.     

The thermodynamic aspect of melting and softening of nanoparticles

The specific features of the energy spectrum of stationary quantum states responsible for the melting and softening of a polyatomic system are investigated. It follows from the first principles of the quantum mechanics and statistical physics that the melting temperature of small nanoparticles can exceed the melting temperature of a macroscopic sample of the same chemical composition. The dependence of the temperature range of the transition of the polyatomic system to a microscopically labile state on the number of atoms is determined.     

Preparation and catalytic properties of composites with palladium nanoparticles and poly(methacrylic acid) microspheres

Palladium (Pd) nanoparticles with different sizes are in situ synthesized by reduction of PdCl₂ with NaBH₄ as reductant in the presence of poly(methacrylic acid) (PMAA)microspheres. The obtained PMAA/Pd composites are characterized by Fourier transform infrared spectra, X‐ray diffraction, and Transmission electron microscopy. The catalytic activity of the PMAA/Pd composites is investigated using a model reaction, that is, reduction of p‐nitrophenol to p‐aminophenol. The reaction shows first‐order kinetics, and the reaction rate increases with increasing reaction temperature, p‐nitrophenol concentration, and loadings of Pd nanoparticles on PMAA microspheres. The PMAA/Pd composites exhibit good stability, ascribing to the Pd nanoparticles stabilized by PMAA microspheres. POLYM. COMPOS., 35:2251–2260, 2014. © 2014 Society of Plastics Engineers     

纳米SiO2粒子对低熔点混合硝酸盐热物性影响

为探究纳米粒子对低熔点混合硝酸盐热物性的影响规律,采用高温熔融分散法将平均粒径20 nm的SiO₂纳米粒子以1%（质量）比例直接分散到混合熔盐[Ca（NO₃）₂·4H₂O-KNO₃-NaNO₃-LiNO₃]中得到不同分散条件下的熔盐纳米复合材料。采用同步热分析仪（DSC）与激光闪射仪（LFA）测量熔盐纳米复合材料比热容与热扩散系数,进而得到热导率。分析发现,600 r/s搅拌速率下熔盐纳米复合材料热物性随分散时间（15,45,90,120和150 min）发生明显变化。比热容、热扩散系数和热导率在分散45 min时提高率最大,平均提高率分别为11.5%,12.9%和26.4%。扫描电镜（SEM）观察到熔盐纳米复合材料表面有大量特殊结构（类似于链状或条状）存在。这些具有高比表面积和表面自由能的特殊结构可能是熔盐纳米复合材料热物性提高的关键。     

Pre-prepared platinum nanoparticles supported on SBA-15 – preparation, pretreatment conditions and catalytic properties

The primary objective of this paper is the complete characterization of Pt/SBA-15 catalysts prepared by sonication aided impregnation. During the experiments (i) the influence of sonication applied for introducing the already prepared platinum nanoparticles into the pores of the silica support, (ii) the pressure used for preparing the tablets and/or wafers for catalytic test, (iii) the temperature of heat treatment on the structural changes of catalyst samples, (iv) the effect of platinum particle concentration, and (v) the removal of rest organic matter from the catalysts by different procedures were systematically studied. The samples were characterized by nitrogen adsorption/desorption measurements (BET), TEM, SAXS, XRD and IR. The catalytic activity of the samples was also investigated in the reaction of cyclohexene hydrogenation.     

Monodisperse platinum nanoparticles of well-defined shape: synthesis, characterization, catalytic properties and future prospects

Monodisperse platinum nanoparticles with well-defined faceting have been synthesized by a modified polyol process with the addition of silver ions. Pt nanoparticles are encapsulated in mesoporous silica during in situ hydrothermal growth of the high surface area support. Removal of the surface regulating polymer, poly(vinylpyrrolidone), was achieved using thermal oxidation-reduction treatments. Catalysts were active for ethylene hydrogenation after polymer removal. Rates for ethylene hydrogenation decreased in accordance with the amount of Ag retained in the Pt nanoparticles after purification. Ag is most likely present on the Pt particle surface as small clusters. Future prospects for these catalysts for use in low temperature selective hydrogenation reactions are discussed.     

Structure and catalytic activity of POSS-stabilized Pd nanoparticles

Palladium nanoparticles stabilized with polyhedral oligomeric silsesquioxanes have been characterized and determined to function as a heterogeneous catalyst. TEM and SEM studies indicate the material comprises 2–5 nm diameter Pd crystallites in a POSS layer, which aggregate to form spherical secondary structures of about 50 nm in diameter. Solid-state NMR reveals that the primary interaction between Pd and POSS is through the alkyl-amino functional groups on the POSS cage, in addition to some weak interactions with the cage itself. This material is observed by microbalance measurements to efficiently catalyze the direct hydrogenation of 1,4-diphenylbutadiyne.     

Structure and catalytic activity of POSS-stabilized Pd nanoparticles

Palladium nanoparticles stabilized with polyhedral oligomeric silsesquioxanes have been characterized and determined to function as a heterogeneous catalyst. TEM and SEM studies indicate the material comprises 2–5 nm diameter Pd crystallites in a POSS layer, which aggregate to form spherical secondary structures of about 50 nm in diameter. Solid-state NMR reveals that the primary interaction between Pd and POSS is through the alkyl-amino functional groups on the POSS cage, in addition to some weak interactions with the cage itself. This material is observed by microbalance measurements to efficiently catalyze the direct hydrogenation of 1,4-diphenylbutadiyne.     

Preparation and catalytic properties of platinum dioxide nanoparticles: A comparison between conventional heating and microwave-assisted method

Using platinum chloride, sodium hydroxide, sodium acetate, and polyvinylpyrrolidone as starting materials, platinum dioxide nanoparticles with small particle diameter and narrow distribution were prepared by microwave irradiation and conventional heating, respectively. UV–vis spectrophotometer was used to trace the hydrolytic decomposition of platinum chloride and the formation of platinum dioxide nanoparticles. X-ray powder diffraction and transmission electron microscopy were employed to characterize the crystalline structure and the morphologies of the obtained nanoparticles. Gas chromatography was performed to investigate their catalytic properties for hydrogenation of cyclohexene. The results revealed that (i) the obtained nanoparticles have the same crystalline structure; (ii) the nanoparticles obtained by microwave irradiation were smaller and more narrowly distributed than those obtained by conventional heating; (iii) the activation time of the catalyst markedly influences the catalytic activity; (iv) the particles obtained by microwave irradiation showed higher catalytic activity than those obtained by conventional heating for hydrogenation of cyclohexene.     

Synthesis of ultrathin carbon dots-coated iron oxide nanocubes decorated with silver nanoparticles and their excellent catalytic properties

A facile ultrasonic method has been successfully developed for the fabrication of multifunctional Fe₃O₄@carbon dot/Ag (Fe₃O₄@C-dot/Ag) nanocubes (NCs), and the resulting materials are well characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), Vibrating sample magnetometer (VSM) and fluorescence measurements. The Ag nanoparticles (NPs) are uniform and well dispersed on the surface of Fe₃O₄@C-dot, while maintaining the shape and the size of the core-shell Fe₃O₄@C-dot NCs. In addition, its catalytic activities are evaluated by measuring the reduction of p-nitroaniline (p-NA) and crystal violet (CV), and the composite materials exhibit excellent catalytic activity towards reduction of p-NA and CV dye, which is superior to most reported catalysts. The good catalytic performance of Fe₃O₄@C-dot/Ag NCs may be attributed to the specific characteristics of its nanostructure and the synergistic effect on the delivery of electrons between Ag NPs and Fe₃O₄@C-dot NCs. Furthermore, the as-prepared catalysts also show good activity for the reduction of other nitrobenzene analogs. The effect of solvent and reducing agent was also studied on the catalytic activity of Fe₃O₄@C-dot/Ag NCs. Most importantly, the Fe₃O₄@C-dot/Ag catalyst shows excellent recycling stabilities, which can be potentially applied in the fields of catalysis and green chemistry.